The invention relates to a three-way catalyst which is formed from two superposed catalytically active layers, is suitable for the cleaning of the exhaust gases of internal combustion engines, and has outstanding catalytic activity coupled with exceptional thermal aging stability.
Three-way catalysts are used in large numbers for the cleaning of the exhaust gases of internal combustion engines operated under essentially stoichiometric conditions. They are capable of converting the three significant gaseous pollutants of the engine, specifically hydrocarbons, carbon monoxide and nitrogen oxides, simultaneously to harmless components. Double-layer catalysts are frequently used, which enable a separation of different catalytic processes and hence an optimal balance of the catalytic effects in the two layers. Catalysts of this kind are described, for example, in EP 1 046 423, EP 0 885 650 or WO 95/35152. When such double-layer catalysts comprise palladium as the catalytically active noble metal, their preparation generally ensures that palladium is present spatially separated from the rhodium which is likewise present as a catalytically active component. For instance, the catalysts described in EP 0 885 650 and WO 95/35152 always comprise rhodium (optionally in the presence of platinum) in the outer, second catalyst layer, whereas palladium (optionally likewise in the presence of platinum) is always present in the inner, first layer. The reason for this usually very careful spatial separation of the noble metals rhodium and palladium lies in the assumption, considered to be valid to date, that these two noble metals, when in direct contact with one another, lose their catalytic activity at the high temperatures typical for three-way catalysts as a result of the formation of intermetallic phases. This would be equivalent to the irreversible thermal deactivation of the catalyst.
In contrast, EP 1 541 220 describes a single-layer, palladium-rich three-way catalyst in which, in the preferred embodiment, at least 70% by weight of the palladium and rhodium present is present in unalloyed state under alloying conditions. The support materials present are preferably zirconium-rich cerium/zirconium oxygen storage materials and γ-aluminum oxide. To prepare the catalyst described, all components used are combined in one suspension and coated onto a support body. In the resulting catalyst, all catalytically active materials are thus present in the spatial proximity of one another.
US 2003/0180197 A1 discloses a catalyst comprising a catalytic metal compound and a macroporous compound, said macroporous compound comprising an oxygen storage material and an aluminum oxide. The catalyst is characterized in that at least 40% of the macropore volume of the oxygen storage material and/or of the aluminum oxide is associated with pores having a pore diameter greater than 120 Å. Apart from a two-layer embodiment of the catalyst with palladium in one layer and rhodium in the other layer, the document also discloses a one-layer catalyst which is produced by coating a substrate with a palladium-containing washcoat and subsequent penetration of the upper 10 micrometers of the coating with a rhodium compound.
The constantly rising demands on reducing the emissions of internal combustion engines necessitate constant further development of the catalysts. Of particular significance are the lightoff temperatures of the catalyst for the conversion of the pollutants and the thermal stability thereof. The lightoff temperature for a pollutant specifies the temperature from which this pollutant is converted to an extent of more than, for example, 50%. The lower these temperatures are, the earlier the pollutants can be converted after a cold start. At full load, exhaust gas temperatures of up to 1150° C. can occur directly at the engine outlet. The better the thermal stability of the catalyst, the closer it can be arranged to the engine. This likewise improves emission control after a cold start.
The catalysts according to the prior art cited already have very good properties with regard to lightoff temperatures and thermal stability. However, the toughened legislation necessitates the search for even better catalysts.